1. Field of the Invention
The present invention relates to an image processing apparatus, such as a copier and printer for forming an image on a transfer member by using an electrophotographic process, and a fixing apparatus incorporated in an image processing apparatus and used to fix a developer to a transfer member.
2. Description of the Related Art
In a copier or a printer using an electronic process, it is known that a toner image formed on a photoconductive drum is transferred to a transfer member, fused in a fixing apparatus including a heating roller and a pressurizing roller, and fixed to a transfer member.
An image processing apparatus of recent years is applicable to various printing modes, such as full-color, cardboard and double-sided printing, and capable of printing at high speed. Large thermal capacity is required for full-color, cardboard and double-sided printing, compared with monochrome, ordinary paper and single-sided printing. However, a low-temperature offset occurs when a heating roller temperature is lower than a fixing temperature, and a high-temperature offset occurs when a heating roller temperature is higher than a fixing temperature. An image is not satisfactorily fixed in either case.
There is a known image processing apparatus as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-259034, for example. The apparatus is capable of exactly and stably controlling a surface temperature of a rotating fixing body, by detecting a surface temperature of a rotating fixing body with good responsivity by using a non-contact surface temperature detection means, and detecting a surface temperature at two more points in a rotating fixing body by one non-contact surface temperature detection means.
Jpn. Pat. Appln. KOKAI Publication No. 2000-066761 discloses a fixing apparatus characterized by non-contact measurement of temperatures of a heating roller or a pressurizing roller at two more points by one temperature sensor, in which paper-passing area and non-passing area of a transfer material are measured easily and exactly by one non-contact temperature sensor.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2001-265156 discloses an apparatus realizing optimum temperature adjustment irrespectively of transfer material sizes, by providing a slit temperature sensor between a fixing roller and a non-contact surface temperature detection means as one body, adjusting minutely the distance from an area definition plate and temperature sensor to a fixing roller by an actuator according to sizes of transfer material passing through a fixing roller, and setting a temperature detection area of a temperature sensor.